semantic names for indicator icons

For all of the discussion that we had about what kind of icons should and
should not be used to indicate what mode the notebook is in, we never went
through to make it possible to override it.

With this change, it is now possible to override what icons are displayed for
Command and Edit Modes.

For example, @minrk liked the fighter-jet icon for Command Mode, so he can put
this in his custom.css

.ipython-command-mode:before {
    content: "\f0fb";
}